cross.bbclass: Remove usage of host flags for cross-compilation
BUILD_* flags can't be used as TARGET_* flags even for "cross" packages.
gcc-cross buils leaks config.log's through "gcc-stashed-builddir" and
TARGET_* flags to libgcc cross-build through "gcc/libgcc.mvars" file
on "gcc-stashed-builddir". This means that if BUILD_CFLAGS contains
host-specific flags like "-isystem/usr/include" libgcc build will
fail "do_qa_configure" and "do_package_qa" checks.
Remove host-related flags from TARGET_* flags for gcc-cross build.
Signed-off-by: Nikolay Merinov <n.merinov@inango-systems.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
